A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is desirable to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
In the manufacture of components using a lithographic apparatus, end of line (EOL) effects can occur. These end of line effects can result in a component feature (such as a channel or gate) that is intended to have a square end being manufactured with a non-square end, such as a tapered, curved, or rounded end.
Typically, a feature of a component being manufactured using a lithographic apparatus is designed to have a length which meets a certain requirement. If the end of line effects described above occur, then the length of the feature can become smaller than the design requirement towards the end of the feature. Where this occurs, the feature effectively becomes shorter than intended. This phenomenon is known as line end shortening (LES), and can have an adverse effect on the component.
For example, in the manufacture of channels for a transistor, the length of the channel is crucial to transistor performance. Thus, if the channel length becomes less than the design requirement due to the end of line effects described above, then the channel effectively becomes shorter, and the transistor performance is affected.
In some instances, it is possible to account for line end shortening that will occur in the manufacturing process during the design process. However, this may require a knowledge of the line end shortening that is likely to occur for particular features in the design. Such knowledge of the extent of line end shortening may be difficult and time consuming to obtain. For example, current methods involve the use of a scanning electron microscope, which is both slow and expensive.
It may also be useful to know the amount of line end shortening that has actually occurred on a particular substrate. For example, it would be useful to know the actual amount of line end shortening that has occurred compared with the amount predicted. This could be used, for example, to verify whether the amount of line end shortening that was predicted to occur during design of a pattern has actually occurred on each substrate. Again, current methods are prohibitively slow and/or expensive.